The invention relates to apparatus and methods for determining the spectral response and the amount of spectral response mismatch between photovoltaic cells, and more particularly, to apparatus and methods for economically, rapidly, and automatically obtaining signals representative of the amount of spectral mismatch between photovoltaic solar cells, wherein problems associated with interconnecting mismatched solar cells together in a common panel are avoided.
It is well known in the photovoltaic solar collector industry that photovoltaic cells, hereinafter referred to simply as solar cells, that are connected in parallel and series relationships in a single solar collector panel need to have closely matched opto-electrical responses to the same spectral distributions, especially that of the sun. Several problems arise if one or more of the individual solar cells are mismatched with respect to the rest of the solar cells interconnected in a single panel. One problem is that if the individual solar cells are not closely matched, it becomes difficult, if not impossible, to specify rated electrical output characteristics for a solar collector panel within a satisfactorily narrow range of accuracy. Another problem is that if one solar cell in the collector panel produces a lower voltage output in response to irradiation by a particular illuminant, that solar cell will conduct more current than the others. This condition is referred to herein as "current hogging", and has several undesirable consequences, one being that the mismatched solar cell overheats, and the other being that the total output power produced by the solar collector panel is decreased. The localized overheating can degrade the reliability and lifetime of the solar collector panel in a number of ways. For example, the overheating of one solar cell may cause gradual darkening or discoloration of the encapsulant of that solar cell, thereby reducing the amount of radiation received by the solar cell's PN junction. As a result of reducing the amount of solar irradiation received by the PN junction, the electrical output thereof is also reduced.
It is also well known that standard reference cells should be spectrally matched to the cells and/or modules being evaluated for electrical performance. This method provides an accurate and simple means of defining the spectral response of any photovoltaic device in terms of two mathematical parameters.
Various techniques have been utilized in the past to characterize and determine the effective amount of "mismatch" between two solar cells. One technique is to illuminate one solar cell through a blue filter, measure the resulting output current, then illuminate that solar cell from the same source through a red filter, and then measure that output. (See Weizer, V. G., "Consideration of Design and Calibration of Terrestrial Reference Solar Cells", document No. NSAS CP-2010, Terrestrial Photovoltaic Measurements. II, Workshop Proceedings, Baton Rouge, La., Nov. 10-12, 1976.) The measured outputs then are ratioed to provide the "red-blue ratio" utilized in the industry to match cells, cells and modules, and modules with each other. If the ratios of corresponding solar cells or devices agree closely, the solar cell under test and the reference solar cell or other device are assumed to be very "closely matched". Unfortunately, this technique has proven to be highly inadequate. Solar irradiance changes a great deal, depending on the time of day, the season, and location on the earth. It has been found that solar cells which appear to be satisfactorily matched according to the above technique are, in fact, highly mismatched for other illuminants, such as the sun. Solar collector panels including individual solar cells that are determined to be matched in the foregoing manner are likely to have all of the abovementioned difficulties that result from use of mismatched devices.
Another proposed method for determining the amount of mismatch between a solar cell under test and a reference solar cell has been to compare the spectral response curves of the two cells or devices in question, one of which may be a reference standard. This is done by normalizing the spectral response curve of the test device to the reference device at 700 nanometer wavelengths. The difference between the integrated normalized spectral response curve of the test device and the integrated spectral response curve of the reference device is convoluted with the Air Mass 1.5 solar spectrum (see the ASTM Committee E44 Draft Document 105 "Terrestrial Direct Normal Solar Spectral Irradiance Tables for Air Mass 1.5", available from The American Society of Testing and Materials, 1916 Race Street, Philadelphia, Pa. 19103) to provide a mismatch parameter. Unfortunately, the evidence available leads to the conclusion that solar cells that are closely matched in accordance with this technique may actually be substantially mismatched in operational performance.
Therefore, there remains an unmet need for a simple method and apparatus for accurately generating a spectral response characterization and a single mismatch coefficient, and an electrical signal representative thereof, to accurately indicate spectral response and the effective amount of mismatch between two photovoltaic cells when they are irradiated by normally varying solar irradiation.
There is a need for such a device that is capable of inexpensive manufacture and utilization and which can be adapted for rapid classification of solar cells during manufacture thereof and sorting the solar cells into groups that are sufficiently closely matched for optimal interconnection in solar collector panels without causing the above-mentioned problems that arise when mismatched solar cells are interconnected in a single solar panel.
Accordingly, it is an object of the invention to provide a method and apparatus for rapid, relatively low cost, accurate determination of the amount of mismatch between two photovoltaic cells.
It is another object of the invention to provide a method and apparatus for rapid, low cost determination of the spectral response of a photovoltaic cell.
It is another object of the invention to provide a method and apparatus for producing a signal representative of the amount of mismatch between solar cells such that solar cells grouped in accordance with predetermined ranges of mismatch as indicated by the signal can be reliably and optimally interconnected in a single solar collector panel that is exposed to the normal range of solar irradiance.
It is another object of the invention to provide a method and apparatus for determining the spectral response of photovoltaic cells or devices in order to facilitate the correct selection of standard reference cells for the accurate assessment of solar irradiance values employed in determining the electrical performance of photovoltaic cells, modules, panels, etc.
It is another object of the invention to provide a method and apparatus for automatically sorting solar cells into closely matched groups during the manufacture of the solar cells.
It is another object of the invention to provide a single parametric description of the mismatch in spectral response between any two photovoltaic cells of the same generic type wherein the parametric description effectively indicates the amount of mismatch between the electrical responses to the two photovoltaic cells over the normal range of solar irradiance.
Users of color difference data have employed summations of three vector-like components of a difference into a single scale or value that is useful for determining whether a specimen color is within a specified tolerance from a standard. See the ASTM publication "Standard Method for Instrumental Evaluation of Color Differences of Opaque Materials", ASTM Standard D2244, pages 415-422, part 27, Paint-, 1981 Annual Book of ASTM Standards, American Soceity of Testing and Materials. Although the technique has proven useful in characterizing color differences, it has never been adopted for use in characterizing differences in electrical characteristics of photovoltaic cells.